One of the known methods for producing weldless tubular elements consists, in a first step, of working a solid profile so as to obtain the hole, by means of a mandrel. The next step consists of reducing the measurement of the section of the tubular element to the desired value. This operation is normally carried out by passing the tubular element through one or more rolling stands, each mainly consisting of at least three rolling rings disposed at 120° with respect to each other and made to rotate by respective shafts. The rolling rings have a peripheral surface shaped as the profile of an arc of a circumference that defines a rolling surface to define part of, or substantially all, the external profile of the tubular element.
This process can mainly cause two types of problem for the quality of the tubular element, both connected to a deformity in thickness.
A first problem concerns a deformity in thickness due to the eccentricity of the mandrel, during the making of the axial hole, with respect to the nominal section of the tubular element to be made.
A second problem concerns a deformity in thickness not due to eccentricity between hole and section but due to localized circumferential variations in the thickness of the tubular element. These variations derive from two situations.
A first situation is connected to the presence or absence of constant contact of the surface of each rolling ring with the external surface of the rolled product.
The second situation concerns the fact that the rolling rings are independent of each other and each ring defines, with respect to the adjacent one, a discontinuity that is mirrored in the development of the surface of the tubular element, in particular on its internal surface, generating a so-called “daisy” section profile of the tubular element.
The deformity in thickness of the final product can therefore derive both from a starting defect, connected to the eccentricity of the mandrel with respect to the nominal sizes of the tubular element, and also from a defect in rolling, connected to the motion and action of the three rolling rings that are independent of each other but work the same piece simultaneously.
In any case, the deformity in thickness has a negative influence on the quality of the whole working, following passes through several rolling stands. Furthermore, the resistance and performance of the tube when in use can also be affected.
Different methods or systems are known to detect the thickness of a tubular element during production and try to make it uniform, adopting correction systems downstream of the rolling line or downstream of a corresponding rolling unit or rolling stand.
One of the known systems provides to use a detection apparatus that works by means of an X-ray radiation principle, and uses one or more transmitter probes and one or more receiving probes, between which the tubular element is made to transit. According to the radiation captured by the receiving probe after the tubular element has passed through, an electric current is generated that is processed and digitized by a measuring transducer and subsequently sent to a central processing system to calculate the thickness of the wall of the tubular element.
Normally, a plurality of transmitter probes and receiving probes are disposed uniformly around the circumference defining the external diameter of the tubular element.
One disadvantage of this technique is its complexity, since the indication obtained for each pair of X-ray probes is the sum of two thickness contributions, corresponding to two opposite parts, or thicknesses, of the cross section of the tubular element, so that it is impossible to know directly and certainly whether the thickness of the tubular element is uniform over the whole circumference, since even if the detections of the probes are identical, the thickness may not be uniform between one part and the other of the tubular element.
Another system provides to use laser and ultrasound technology. In this case, a transmitter probe emits a pulsing laser that generates an ultrasonic wave which propagates from outside to inside the thickness of the tubular element, is subsequently reflected by the internal surface of the tubular element and then returns toward its external surface. A laser interferometer, inspecting the external surface of the tubular element, determines the time taken by the ultrasonic wave to twice pass through the thickness of the tubular element, that is, in an out-and-return travel. Since the speed of propagation of the ultrasonic wave is known, the system is able to deduce the thickness of the tubular element as a function of the passage time measured.
In this case, the detection refers to the portion of thickness cooperating with the probes.
This detection technique also provides the movement and partial rotation of the support parts of the probes, for example by means of automatic mechanical arms, to obtain the detection of the measurement on several surface portions of the tubular element so as to substantially cover the whole circumference thereof.
One disadvantage of this known technique is the sensitivity of the detection with respect to the position of the transmitter probe and the receiving probe. In fact any displacement thereof, even by a small value, can give an erroneous thickness indication.
Furthermore, the devices described above are in general rather expensive and difficult to manage and to maintain.
Another detection system is similar to the previous one but provides to use an electromagnetic transducer which, since it is able to generate magnetic field impulses, causes ultrasonic waves that are reflected by the surfaces of the tubular element and captured by a receiving probe.
In the last two cases cited, the detection systems also have the disadvantage of being very sensitive to the positioning both of the probes and of the tubular element, which negatively affects reliability.
All the systems described, moreover, entail possible problems in setting and adjusting the position with respect to the axis of the tubular element being detected, so that even a slight dis-axiality can entail quite considerable errors.
Document DE 10 59 672 B (DE'672) describes a device for the continuous control of the thickness of the wall of a non-metal pipe, for example made of plastic, in which at the point of measurement the pipe is supported by an internal support made of ferromagnetic material.
The measurement is obtained by means of unipolar probes, for example 3, disposed at 120° around the circumference of the pipe. Each of the probes, measuring the auto-induction, that is, the angle of loss of a measuring coil, allows to obtain the value of thickness of the wall in contact with the corresponding probe.
The solution described in DE'672 is not suitable to continuously measure the thickness of metal pipes during rolling.
One purpose of the present invention is therefore to obtain an apparatus, and the corresponding method, to detect the deformity in thickness of metal tubular elements, which provides reliable measurement results and which is relatively simple in installation and in functioning.
Another purpose of the present invention is to obtain an apparatus that reduces the incidence of errors deriving from the positioning of the detection elements with respect to the metal tubular element.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.